Otra manera:
Código PHP:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
</head>
<body>
<img id="im" src="loquesea.jpg" />
<script>document.getElementById('im').src+='?'+(+new Date);</script>
</body>
</html>